Project Summary/Abstract Human anaplasmosis, caused by the obligate intracellular bacterium Anaplasma phagocytophilum, is one of the most common tick-borne diseases in the United States. In mammals and ticks, A. phagocytophilum resides in neutrophils and in salivary glands, respectively. Despite numerous studies that have focused on understanding strategies that A. phagocytophilum uses to survive in the mammalian cells, relatively few studies have clearly defined the molecular strategies that A. phagocytophilum uses to survive in ticks. In this project, we will be performing a comprehensive molecular analysis on Ixodes scapularis organic anion transporting polypeptides (OATPs) and genes involved in the tryptophan metabolism pathway in A. phagocytophilum-tick interactions. This study is build upon our efforts in showing how A. phagocytophilum modulates gene expression and cell signaling for its survival in the vector. As an example, our previous studies has shown that A. phagocytophilum modulate tick antifreeze gene expression and actin phosphorylation for its survival in the vector. Here, we provide strong preliminary in vitro and in vivo data showing that A. phagocytophilum induces expression of a specific OATP (OATP4056) and kynurenine aminotransferase (KAT), a gene involved in the tryptophan pathway. We now hypothesize that interplay between OATP4056 and KAT not only facilitates A. phagocytophilum survival in the vector but also aid in the transmission of this bacterium to a vertebrate host. RNAi analysis revealed that knockdown of OATP4056 has no effect on A. phagocytophilum acquisition but affected its survival and transmission from these ticks. Electrophoretic mobility shift assays with promoter region and total lysates prepared from uninfected and A. phagocytophilum-infected ticks provide further evidence that the presence of this bacterium influences expression of OATP4056 in these ticks. In addition, we found evidence that a metabolite from tryptophan pathway regulates A. phagocytophilum survival and OATP4056 gene expression in these ticks. These results provide important insights for our proposed studies to define molecular basis of the relationship of A. phagocytophilum with ticks. Based on our strong preliminary results and the experiments proposed, we believe that this could be a transformative study that not only serves as a model to study intimate relationships established by pathogens with their arthropod vectors but may also lead in the development of new strategies to interrupt the transmission of this and perhaps other Rickettsial species of medical importance.